For an Internet communication system such as Voice Over IP (“VoIP”) and an instant messaging system, Session Initiation Protocol (“SIP”), such as a protocol describe in the Request for Comments 3261 (“RFC 3261”) by The Internet Society in The Internet Engineering Task Force, which is herein incorporated by reference, is generally used as a signaling protocol of an application layer level for establishing, maintaining, and terminating a multi-media session. A communication device in a system using SIP communicates location information such as an IP address of the communication device by transmitting a REGISTER request to a register server of the system. A REGISTER message, in addition to the location information of the communication device, can include time information such as an “expires” parameter. For example, the expires parameter can limit a use of the communication device in the system and allow operating in the system only during an allowed period such as a normal work period from 9:00 AM to 5:00 PM, or a specific duration such as one hour. A user may register a plurality of location information, such as several IP addresses which the user can use in the system and may request the system to search all of the plurality of location information until one is found, indicating the user is present at one of the IP addresses. The registration of the communication device may be revoked or expired during the allowed period by transmitting a predetermined parameter to the system.
For a wireless local area network (“WLAN”) system 100 of FIG. 1, a WLAN coverage area 102 is served by a base station 104, or an access point, which is connected to a network 106, which is a land-line network. The WLAN system 100 further comprises a register server 108, which is also connected to the network 106, and a location server 110, which is connected to the register server 108. In this example, the register server 108 functions also as a SIP proxy server defined in SIP. A proxy server assists in various aspects of SIP communication handling such as routing requests to a user's current location, authenticating and authorizing users for services, implementing provider call-routing policies, and providing features to users. When a wireless mobile communication device 112 detects a wireless signal 114 from the base station 104, the wireless mobile communication device 112 communicates with the base station 104 wirelessly, and transmits a REGISTER message 116 to the base station 104. The base station 104 then communicates the REGISTER message 116 received from the wireless mobile communication device 112 to the register server 108 through the network 106. The register server 108 interprets the REGISTER message, and associates the location information, i.e. IP address, of the wireless mobile communication device 112 to a SIP address such as a Uniform Resource Identifier (“URI”) of a machine into which the wireless mobile communication device 112 is currently logged. The register server 108 then registers the location information of the wireless mobile communication device 112 to the location server 110.
When making a telephone call using SIP, the wireless mobile communication device 112 transmits an INVITE request, which includes location information of a target user or device, to the base station 104 which forwards the INVITE request to the register server 108. The register server 108 then searches for identified in the INVITE request based on registered information. SIP can also provide a presence service, which notifies users of a status, active or inactive, of a particular user based upon the registered information.
In a system supporting a specific length for the expires parameter upon registering, a registered communication device, wired or wireless, needs to periodically transmit a REGISTER message before the expires parameter expires in order to remain active in the system beyond the initially granted allowed time period. For example, if the system sets the expires parameter to be one hour upon registration, then the communication device needs to transmit a REGISTER message before the end of the one hour if the communication device is to be active beyond the initial one hour period. The registered information such as location information becomes invalid at the expiration of the expires period. However, as shown in FIG. 2, if the wireless mobile communication device 112, which is registered in the system, leaves a coverage area 102 supported by the base station 104 during an expires period, the registered location information of the wireless mobile communication device 112 is still considered to be valid by the system because the expires period has not expired. Therefore, the system will assume that the wireless mobile communication device 112 is still active in the system or is within the coverage area 102, and will provide valid presence information of the wireless mobile communication device 112 even though the wireless mobile communication device 112 cannot be reached. Further, because the wireless mobile communication device 112 is out of the coverage area 102, the wireless mobile communication device 112 is unable to notify the system that the system needs to un-register the wireless mobile communication device 112. If another communication device attempts to call the wireless communication device, which is beyond the coverage area but still has registered location information, the system will find valid location information, i.e. the IP address, of the wireless mobile communication device 112 and will attempt to complete the call. Because the system goes through all steps for establishing a call to the wireless mobile communication device 112 that is not reachable, it takes time for the system to time out the call, and a response to the communication device originating the call is not immediate.